Hot Corrosion Behavior of TWAS and HVOF NiCr-Based Coatings in Molten Salt

Abstract

In order to extend the life of boilers by applying an anti-corrosion coating without the need to dismantle them, it is advisable to find coatings that can be applied using cheaper and portable techniques, such as Twin Wire Arc Spray technology (TWAS). In this study, we compare selected NiCr-based coatings and two uncoated steel substrates (steel 1.7715 and 1.4903). Two coatings, Cr3C2 - 25% NiCr and Hastelloy C-276 are deposited using High velocity oxygen-fuel technology (HVOF) and three coatings, NiCrTi, NiCrMo, and Inconel 625, are deposited using TWAS. In addition to the corrosion weight gain during 50 cycles of loading in an 18% Na2SO4 and 82% Fe2(SO4)3 salt environment at 690 °C evaluated using the gravimetric method, the microstructure and phase composition of the coatings were analyzed on the samples after the exposure in order to compare the properties and gain a deeper understanding of the corrosion kinetics. Coating cross-sections and free-surfaces were observed with a scanning electron microscope (SEM) with an energy-dispersive (EDX) system. The phase composition was investigated using X-ray diffraction (XRD) and Raman spectroscopy. No significant differences were observed between the TWAS and HVOF coating methods for the coatings compared. Due to the similar corrosion products found on all coatings, a very effective corrosion protective layer was formed on the surface, forming a barrier between the corrosive environment and the coating regardless of the used deposition technology. Therefore, for industrial use on the inner surface of coal-fired boilers we recommend NiCrTi, NiCrMo, or Inconel coatings prepared with the more cost-effective and portable TWAS technology.

WITHDRAWN: Hot Corrosion Evaluation of Cr 3 C 2 /8YSZ based Plasma Sprayed Coatings on SS316 in Na 2 SO 4 – V 2 O 5 Molten Salt Environment at 700 0 C

“…Deepa Mudugal et al [8] & [9] performed the Cr 3 C 2 -NiCr coating on Superni 600 alloy and ascertained that Cr 3 C 2 -based feedstock powders are the right choice for coatings in components used in high-temperature erosion and corrosion environments. The authors reported in [10][11][12][13] studied the the effects of elevated temperatures on chromium-based coatings applied to boiler steels and nickel-based superalloys, particularly when exposed to a corrosive hot salt environment and found the accumulation of NiO and Cr 2 O 3 oxide layers on the coated surface. The existance of corrosion layer of oxide serves as an inhibitor to hinder the in ltration of the corrosive medium, hence impeding the oxidation of the entire coating.…”

High-Temperature Corrosion Behavior of Selected HVOF-Sprayed Super-Alloy Based Coatings in Aggressive Environment at 800 °C

“…The durability and reliability of power generation equipment are important parameters affecting the stability of the energy market. In the case of components of combustion plants, such as boiler firewalls, heat exchangers, or pressure vessels of coal-fired power plants, the service life is often limited by the corrosion attack of an aggressive environment, where hot corrosion is especially a major issue determining the service life of said components [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ]. Hot corrosion has been identified as a serious problem for many high-temperature aggressive environment applications, such as boilers [ 1 , 2 , 4 , 5 , 6 , 7 , 8 , 11 , 12 , 14 ], internal combustion engines, gas turbines [ 15 , 16 ], fluidized bed combustion boilers [ 17 ], industrial waste incinerators [ 18 , 19 ], etc.…”

“…The most dominant substance known to promote hot corrosion processes is Na 2 SO 4 , mainly owing to its very good temperature stability in a wide range of oxygen partial pressures [ 7 ]. Na 2 SO 4 originates in oxidation reactions during the combustion of fuel containing both sulphur and sodium and possibly other alkali metals, such as potassium, which also form another corrosively aggressive sulphate attributed to the hot corrosion process—K 2 SO 4 [ 1 , 7 , 8 , 10 , 20 ]. Some other major aggressive substances known for their negative attribution to hot corrosion process are chlorides and vanadates [ 7 , 8 ].…”